The present invention relates to a sputtering process for forming a thin film and, more particularly, to a sputtering process capable of satisfactorily coating minute stepped parts, minute grooves and minute holes in the surface of a substrate for a highly integrated multilayer semiconductor device having minute wirings, and an apparatus carrying out the method.
Japanese Patent Provisional Publication No. 60-221563 discloses a bias sputtering apparatus designed to apply a strong magnetic field to a plasma to increase current flowing into the substrate on which a film is to be formed. That is, in this bias sputtering apparatus, an objective substrate is disposed opposite to a magnetron sputtering electrode, and part of the ions in a plasma generated on the magnetron sputtering electrode is caused to flow into the surface of the substrate by applying a negative bias voltage to the surface of the substrate.
U.S. Pat. No. 3,325,394 discloses a sputtering technique employing a cusp field. According to this technique, an objective substrate is disposed opposite to a sputtering electrode, and a cusp field is produced between the objective substrate and the sputtering electrode by two sets of electromagnets to increase the density of the plasma and to increase the film forming speed. Since a magnetron sputtering process, which was disclosed after the sputtering process employing a cusp field, is simpler in the construction of the apparatus for carrying out the same and more effective than the sputtering process employing a cusp field for increasing the film forming speed, no attention has been paid to the use of a cusp field.
In the bias sputtering process, a negative bias voltage is applied to the surface of a substrate while a film forming material is deposited over the surface of the substrate in order that ions of a plasma fall on the surface of the substrate to give the energy of the ions to the particles of the film forming material in order to enhance the movement of the particles of the film forming material over the surface of the substrate so that the spread of the particles of the film forming material over the surfaces of grooves and holes formed in the substrate is improved. Accordingly, it is essential to increase an ionic current flowing into the substrate by increasing the quantity of ions which fall on the substrate, which can be attained by increasing the plasma density on the substrate, or by increasing the bias voltage.
Since the conventional magnetron sputtering electrode retains a plasma on the surface thereof facing the substrate, plasma density on the surface of the substrate cannot be increased to a sufficiently high degree simply by applying a bias voltage to the surface of the substrate. Our preliminary experiments showed that the intensity of ionic current flowing into the substrate was on the order of 0.5 A/125 diam., plasma density on the substrate was on the order of 2.times.10.sup.10 /cm.sup.-3 and the spread of the film forming material (aluminum) over the surface of a square hole of 1.0 .mu.m square and 1.0 .mu.m in depth.
As generally known, the quantity of Ar gas absorbed by the film formed by sputtering increases with bias voltage.
It was also found through our preliminary experiments that voids and blisters appear in a film formed by sputtering using a high bias voltage on the order of 140V or above, when the film is subjected to annealing after sputtering.